In ophthalmic surgery, two different techniques are known for cornea transplants.
Penetrating (or full-thickness) transplants entail removal of the entire thickness of the patient's corneal tissue and complete replacement with a donor cornea. By an appropriate device, a circular portion of the recipient patient's central cornea is removed by complete penetration. The tissue removed is fully replaced by a lenticle of equivalent geometry, obtained from the donor cornea. The operation concludes with suture (with different techniques possible) around the perimeter of the transplanted tissue. Following a penetrating transplant, the patient uses only the donor portion of cornea for vision.
In the case of lamellar transplants, on the other hand, only a certain thickness of corneal tissue is removed from the epithelial layer and/or from the endothelial layer of the recipient patient, using one of several surgical techniques available. Lamellar transplants are classified, according to the residual thickness of the recipient tissue, as DALK (Deep Anterior Lamellar Keratoplasty) if the removal is deep and the residual tissue thickness of the recipient is modest, and ALK (Anterior Lamellar Keratoplasty) if the removal is more superficial and the residual tissue thickness is greater. Lamellar transplants also comprise endothelial transplants. In this case the corneal lamella of the donor refers to the rear portion of stroma on which the endothelial cells are arranged.
The removal of material allows a seat to be obtained, normally called receiver stromal bed or more simply receiver bed, in the patient's stromal tissue. The receiver stromal bed is shaped to receive a lenticle or stromal flap from a donor.
Regardless of the surgical technique adopted for preparation of the receiver bed, the grafting of the donor stromal flap creates an interface between the tissue of the recipient and the tissue of the donor. The interface influences the properties of the resulting optical system.
In a significant number of cases, a cornea transplant, whether lamellar (anterior or posterior) or penetrating, although correctly performed, does not give the patient the expected refractive results in terms of improvement of the quality of vision. In particular, and this is not uncommon, evaluation of the transplant performed by the usual objective ophthalmic instrumental examinations may not correspond to the subjective perception of the patient, who does not notice the predicted improvements. Post-operative examinations show that, at times, the quality of vision is not satisfactory even when both the receiver structure and the donor flap are perfectly transparent according to the traditional ophthalmic examination procedures. It has further been found that the visual result is all the more predictable the greater the quantity of tissue removed from the cornea of the recipient and in the case of endothelial transplants, the thinner the stromal lamella of the donor.
Especially in the case of lamellar transplants, the properties of the final optical system are therefore determined not only by the quality of the residual receiver tissue and the donor tissue, but also by the way in which the donor flap is coupled to the receiver bed. However, analogous problems have been encountered also in the case of penetrating transplants.
The problem of determining the correct orientation of the donor cornea is therefore of a general nature, even if perceived more acutely in the case of lamellar transplants.